1. Field of Invention
This invention is in the field of laser-based defect localization analysis of integrated circuits (IC). More specifically, this invention is about design debug and/or failure analysis of ICs using the laser assisted device alteration (LADA) technique.
2. Related Art
LADA (Laser-Assisted Device Alteration) is a technique that depends on the ability of a continuous-wave (CW) laser to generate localized photocurrents in an integrated circuit through its backside and thus change the pass/fail outcome of a test stimulus on a “sensitive” transistor, thereby localizing sensitive areas including design or process defects. The laser is used to temporarily alter the operating characteristics of transistors on the device. The current spatial resolution using the 1064 nm continuous wave laser is 240 nm.
An explanation of the LADA techniques can be found in, for example, Critical Timing Analysis in Microprocessors Using Near-IR Laser Assisted Device Alteration (LADA), Jeremy A. Rowlette and Travis M. Eiles, International Test Conference, IEEE Paper 10.4, pp. 264-273, 2003. That paper described the possibility of using a CW laser of 1064 nm or 1340 nm wavelength. It is explained that the 1340 nm would cause device alteration via localized heating, while the 1064 nm would cause device alteration via photocurrent generation. It is also noted that the 1064 nm laser has a spatial resolution advantage. Therefore, the authors recommend the use of 1064 nm laser.
As shown in FIG. 1, conventional LADA uses a CW laser to induce electron-hole pairs in the device under test from the backside. The electron-hole pairs so generated affect the timing of the nearby transistor—thus facilitating critical path analysis. A DUT 110 is coupled to a tester 115, e.g., a conventional Automated Testing Equipment (ATE), which is connected to computer 150. The ATE is used in a conventional manner to stimulate the DUT with test vectors and study the response of the DUT to the test vectors. The ATE can be replaced by a system test board, optionally controlled by a PC or such. Therefore, sometimes reference will be made to Testing Equipment (TE) as shorthand for either an ATE or other means for testing the DUT. Conversely, all references to ATE incorporate the use of other testing means as well. The response of the DUT to the test vectors can be further investigated using the LADA. For example, if the DUT fails a certain test, LADA can be used to investigate whether the DUT can pass under certain conditions and, if so, which device, i.e., transistor, was responsible for the failure. Conversely, when the DUT passes certain tests, LADA can be used to investigate under which conditions the DUT will fail these tests and, if so, which device, i.e., transistor, was responsible for the failure.
The LADA system for FIG. 1 operates as follows. Tiltable mirrors 130 and 135 and objective lens 140 are used to focus and scan a beam from CW laser 120 onto the DUT 110. This allows the laser 120 to generate photo carriers in the silicon of the DUT 110. The electron-hole pairs so generated affect the timing of the nearby transistor, i.e., decreasing or increasing transistor switching time. The tester is configured to place the operating point of the device under test in a marginal state by applying a recurrent test loop of selected voltage and frequency. The laser stimulation is then used to change the outcome of the tester's pass/fail status. The beam's location at each point is correlated to the pass/fail outcome of the tester, so that when a change is detected, i.e., a previously passing transistor is now failing or vice versa, the coordinates of the laser beam at that time points to the location of the “borderline” transistor.
During the LADA analysis, the tester (ATE) is configured to place the operating point of the device under test in a marginal state. The laser stimulation is used to change the outcome of the tester's pass/fail status. The present state-of-the-art in laser assisted fault spatial localization is about 240 nm resolution. The limitation on further improvement of the single photon LADA spatial resolution is imposed by the laser light wavelength. As noted in the Rowlette paper, the spatial resolution is enhanced by using shorter wavelength. However, optical absorption of silicon at wavelength smaller than 1064 nm prevents the use of shorter wavelengths, as it becomes the major obstacle for delivering light to the transistor through the backside. Thus, while design rules of modern devices shrink, the spatial resolution of the LADA system cannot be improved by the use of smaller wavelength laser. For example, at 22 nm design rule it is doubtful that conventional LADA will be able to resolve among four neighboring transistors.
Optical beam induced current (OBIC) is another test and debug analysis technique in which laser beam is illuminating the DUT. However, unlike LADA, OBIC is a static test, meaning no stimulus signal is applied to the DUT. Instead, the laser beam is used to induce current in the DUT, which is then measured using low-noise, high-gain voltage or current amplifiers. OBIC has been used in a single-photon mode and in a two-photon absorption mode, sometimes referred to as TOBIC or 2P-OBIC (two-photon optical beam induced current).
Two-photon absorption (TPA) is the simultaneous absorption of two photons of identical or different frequencies in order to excite a molecule from one state (usually the ground state) to a higher energy electronic state. The wavelength is chosen such that the sum of the photon energy of two photons arriving at the same time is equal to the energy difference between the involved lower and upper states of the molecule. Two-photon absorption is a second-order process, several orders of magnitude weaker than linear (single-photon) absorption. It differs from linear absorption in that the strength of absorption depends on the square of the light intensity, thus it is a nonlinear optical process.